gain of bond orderNOTE loss of bond order 3THE DIENE MUST BE ABLE TO ADOPT THE S-CIS CONFORMATIONrotate

reactss-trans conformations-cis conformationdoes not reactreacts normally 4DIELS-ALDER REACTIONHOMO(diene)The reaction is concerted - all of the orbitals are aligned in a 6-ring.2The HOMO of the diene donates elec- trons into the LUMO of the dienophile.1LUMO(dienophile) 5BUTADIENEETHYLENEp4The HOMO of the diene donates electrons into the LUMO of the dienophile.p3LUMOHOMOp2ppp1 6EXAMPLE - WITH ELECTRONIC FACTORSpullpushDDieneDienophileA CyclohexeneWorks best if the dienophile has electron-withdrawing groups, and the diene has electron-donating groups.The HOMO of the diene donates PUSHES electrons into the LUMO of the dieno- phile PULLS. 7FORMATION OF A BICYCLIC COMPOUNDD 8THE REACTION IS USUALLY STEREOSELECTIVEendoexoThe endo product is usually preferred.It has been suggested that the pi systems like to establish maximum overlap during the reaction. 9MORE DIELS-ALDER REACTIONSDD 10Retrosynthesis 11ACETYLENES (ALKYNES)If the dienophile is an alkyne a cyclohexadiene is formed. 12CARBENES - CYCLOPROPANES 13a-ELIMINATIONSREVIEW( also covered in Section 9.13 )Alkyl halides which have no b-hydrogens willundergo a-elimination to give carbenes.b-hydrogenno b-hydrogens in these groupsaaba-hydrogenE1 or E2carbene-alkene 14MECHANISMSome halides without a-hydrogen atomsStep 1fast-Step 2-..slow

..-carbeneis a strong base such as NaOH, NaOEt, KOH, NaNH2, or nBuLi 15CARBENES ARE ELECTRON-DEFICIENTCarbenes are neutral, no charge, but have an incomplete octet.They react with alkenes (electron donors) to form cyclopropanes.carbeneRRsyn additionciscisRRa cyclopropaneThe reaction is thought to be concerted.STEPWISE ANALYSIS OF THE CONCERTED REACTION..

-

16SOME REACTIONS OF DICHLOROCARBENE

17SYNTHESIS OF NORCARANEphenyl sodium-

-strong base

ylide--a -eliminationtetramethylammonium chloride

carbenenorcarane(bicyclo 4.1.0 heptane 18explosive if not handled carefullyDIAZOMETHANE(gas)UNSTABLEeasily decomposed by light or heat-....-

light heatphotolysispyrolysis

hn D

nitrogen is a very thermodynamically stable molecule, and forms readilynitrogenmethylene( the specific name for this carbene ) 19DIAZOMETHANE REACTIONSpyrolysisD

Zn-Cu is a zinc-copper couple which is more reactive than either would be alone. Copper activates the zinc.RR

cisRRcisconcertedstereospecificcarbenoid reactions give a result as if a carbene is involved, but it is not. 22WHY USE THE SIMMONS-SMITH ?1) not hazardous like diazomethane2) milder reaction conditions3) doesnt require strong base like a-elimination4) fewer side products5) higher yields.. what else could you want ?

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